The present invention relates to brakes used on, for example, commercial truck or trailer axles, and in particular to automatic slack adjusters which eliminate excess motion in a brake actuator mechanism used to apply the brake.
Over the life of the brake linings of a brake, such as a pneumatic drum brake used on commercial vehicle axles, as the brake's friction linings wear the clearance between the brake linings and their respective friction surfaces (for example, the inner surface of a brake drum) increases. This increasing clearance requires an ever-increasing range of motion from the brake actuator mechanism to move the brake linings from their rest position to the point at which the linings contact the friction surface.
It has become commonplace to include an automatic slack adjuster in the mechanical path between the brake actuator and the brake linings so as to eliminate excess lining travel slack as the brake linings wear. Such adjusters typically are: (i) located on a portion of a brake camshaft which is outside of the brake (typically splined to the camshaft); and (ii) coupled to a pushrod of a brake actuator such that when the brake actuator push rod is extended or retracted, the slack adjuster rotates about the longitudinal axis of the brake camshaft. An example of such a brake and slack adjuster arrangement is shown in FIG. 1 of U.S. Pat. No. 4,380,276. Thus, by extending or retracting the brake actuator pushrod, the slack adjuster causes the brake camshaft to rotate about its longitudinal axis, which in turn rotates a brake actuation cam affixed to the end of the brake camshaft located within the drum brake. The rotation of the cam either presses the brake linings into engagement with the brake drum inner friction surface or allows the brake linings to withdraw radially inward, away from the friction surface. Because the brake camshaft is used to rotate the cam which presses the brake linings radially outward, the brake camshaft is also known as the brake cam.
Automatic slack adjusters can be designed to transmit brake actuator force to the brake camshaft in the brake application direction with no relative motion between the adjuster and the brake camshaft. When the brake actuation force is withdrawn, if there is greater than desired distance between the brake linings and the brake drum friction surface, the slack adjuster is permitted to rotate relative to the brake camshaft an angular distance sufficient to remove some or all of this undesired slack, i.e., limiting the distance the brake linings withdraw from the brake drum friction surface so that the lining-drum clearance is maintained at a desired minimum.
Automatic slack adjusters as described above, where the slack adjuster rotates relative to the brake camshaft when the brake actuation force is withdrawn, are said to adjust on release. There is also the other category of automatic slack adjusters which rotate relative to the brake camshaft during the phase when the actuation force is applied, with no relative rotation when the actuation force is withdrawn, this category being said to Adjust on Apply.
In many automatic slack adjusters, a one-way clutch is used to accomplish the rotary adjusting movement, with a worm shaft located in the adjuster turning a worm gear (also known as a worm wheel) coupled to the brake camshaft. In the Adjust on Release type, when the brake actuator pushrod is retracted, the worm shaft of the worm gear set rotates about is longitudinal axis, causing the worm shaft to move relative to the worm gear in a circumferential direction about the circumference of the worm gear. This relative movement of the worm shaft and gear creates corresponding relative motion between the slack adjuster body and the brake camshaft. As a result, when the brake actuator pushrod returns to its rest position the brake camshaft does not return to its original rest position. Instead, the brake camshaft only rotates through a smaller angle to a new rest position. The brake application cam thus stops in a corresponding new rest position at which the brake linings are maintained closer to the brake drum friction surface. In the Adjust on Apply type, when the brake force is applied, the worm shaft of the worm gear set rotates about is longitudinal axis, causing the worm shaft to move relative to the worm gear in a circumferential direction about the circumference of the worm gear. This relative movement of the worm shaft and gear creates corresponding relative motion between the slack adjuster body and the brake camshaft. As a result, when the brake actuator pushrod returns to its rest position the brake camshaft does not return to its original rest position. Instead, the brake camshaft only rotates through a smaller angle to a new rest position. The brake application cam thus stops in a corresponding new rest position at which the brake linings are maintained closer to the brake drum friction surface. Because the rotation of the slack adjuster relative to the brake camshaft results in reduction of brake lining clearance in the new rest position, the automatic slack adjuster compensates for brake lining and drum wear.
In one type of one-way clutch arrangement, the one-way clutch is coupled to the worm shaft through a toothed clutch, which may conically shaped. A heavy coil spring or disc-spring pack is positioned at the opposite end of the worm shaft to keep the conical clutch engaged and to provide adequate torque to turn the worm shaft. The worm shaft turns the worm wheel, which is coupled to brake camshaft, in order to decrease the brake lining clearance and thus compensate for lining wear. Examples of such arrangements are shown in prior art FIGS. 1-3, corresponding respectively to FIG. 4 of U.S. Pat. No. 4,380,276 (toothed clutch teeth 63), FIG. 3 of U.S. Pat. No. 5,327,999 (toothed clutch 8), and FIG. 1 of U.S. Pat. No. 5,664,647 (toothed clutch 14).
Typically, an external extension of the worm shaft projects outside the automatic slack adjuster housing to permit manual brake lining clearance adjustment during the installation of the slack adjuster or of new brake linings (in FIG. 1, extension 57; in FIG. 2, extension 4′; in FIG. 3, extension 15). The extension usually is shaped as a square or hexagon to facilitate gripping and turning with a wrench or other tool. In order to advance the brake lining, the worm shaft must be rotated in a first direction (designated the clockwise direction for the purpose of this description). In order to retract the brake lining, the worm shaft must be rotated in the opposite, or counter-clockwise, direction.
When the external extension is rotated in the clockwise (advance) direction, the toothed clutch remains engaged, and the worm shaft rotates with little resistance from the one-way clutch permitting the worm shaft to rotate with little resistance. When the external extension is rotated in the counter-clockwise (retracting) direction, the one-way clutch is rotated in its “lock-up” direction, and therefore the toothed clutch coupling strongly resists rotation of the worm shaft. The strong resistance requires application of high torque loads to the external extension, up to the point at which the toothed clutch begins to slip, disconnecting the one-way clutch from the worm shaft.
The slipping of the toothed clutch in response to the application of a large torque to the external extension results in undesired blunting of the teeth in the clutch. As the clutch teeth wear, the torque capacity of the automatic slack adjuster decreases, progressively reducing the useful service life of the automatic slack adjuster. Attempts have been made to reduce this undesired deterioration of the clutch teeth, for example, by altering the angle of the clutch teeth or rounding the tips of the teeth as shown in prior art FIGS. 3a-3b, corresponding to FIGS. 3-4 of U.S. Pat. No. 5,664,647. However, these slight teeth geometry changes have not been fully successful in addressing the wear concerns.
In view of the foregoing, it is an objective of the present invention to provide an improved automatic slack adjuster with superior manual adjustment provisions. In addressing these and other objectives, the present invention provides a solution to the problems of the prior art by providing for an automatic disengagement of the one-way clutch teeth and free release of the clutch to permit smooth withdrawal of brake shoes as an external adaptor part is manually operated.
In one embodiment of the present invention, the adaptor part and an adjacent end of the automatic slack adjuster's worm shaft are provided with corresponding axially-oriented lugs which permit the adaptor part to drive rotation of the worm shaft in the clockwise and counter-clockwise directions. Between the lugs, ramps are provided which, when the lugs are displaced circumferentially relative to one another, cause the adaptor part to push the worm shaft to move axially away from the adapter part. This axial displacement of the worm shaft axially lifts the output part of the one-way clutch out of engagement with the input part, disengaging the one-way clutch's one-way teeth and thereby permitting the worm shaft to smoothly rotate within the automatic slack adjuster housing without damaging the engagement teeth within the one-way clutch.
In an alternative embodiment of the present invention, a similar camming action may be obtained by the interaction of camming surfaces of the adaptor part and a rod which extends through a bore in the worm shaft and is fixed to the output part of the one-way clutch. In this embodiment, when the adaptor part is operated in the brake shoe retraction direction, the axial displacement of the cam rod pushes the one-way clutch output part out of engagement with the input part, freeing the worm shaft to be smoothly turned.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.